Accumulating evidence suggests that there is a strong mitochondrial component in cardiac physiology and pathophysiology. Heart failure (HF) is a condition commonly seen in elderly;it can compromise the quality of a patient's life and it is associated with an increased rate of mortality. Currently, HF is recognized as a disease that occurs due to aberrant cardiac energy metabolism. The cardiac mitochondrion is considered the central integrator of myocyte metabolism and it may be possible to ameliorate HF progression through the direct stimulation of mitochondrial function. The molecular mechanisms that regulate mitochondrial function are gradually being uncovered but the critical link between mitochondrial morphology and mitochondrial function is poorly understood. Mitofusin (Mfn) 1 and 2 are two recently discovered mitochondria-shaping proteins that are found on the outer mitochondrial membrane. Research using a variety of experimental approaches shows that Mfn1 and Mfn2 are major regulators of mitochondrial architecture. Interestingly, Mfn1 and Mfn2 are robustly expressed in heart but so far there are no reports that describe their specific role in this tissue. In the proposed studies, we hypothesize that Mfn1 and Mfn2 are produced by the cardiac myocytes where they perform crucial roles in the maintenance of proper architecture and function of the mitochondrial compartment and that their perturbation will have profound and detrimental effects on heart performance. To address these possibilities we will generate Cardiomyocyte-specific Knockout mice for Mfn1 and Mfn2 (CKOMfn1, CKOMfn2). These mice will be used to assign the physiological function of Mfn1 and Mfn2 in the intact heart. Adult myocytes will be isolated from CKOMfn1, CKOMfn2 and control hearts to assess contractility and calcium handling. Adult myocytes from these strains will also be used to rigorously assess mitochondrial dynamics and function. Collectively, the proposed study will assess for the first time the importance of Mfn1 and Mfn2 in cardiac myocytes using powerful mouse genetic reagents. These experiments will provide a comprehensive top-down analysis of mitofusin function in the heart linking the intact heart phenotype with isolated cardiac myocyte function and mitochondrial function and dynamics. The outcome of this research will provide important information about mitochondrial fusion in the myocardium. PUBLIC HEALTH RELEVANCE In heart, organelles referred to as mitochondria have critical roles in energy production, calcium homeostasis and the generation of reactive oxygen species. It is now recognized that mitochondria are dynamic structures that undergo continuous fission and fusion. To understand the role of mitochondrial dynamics in heart function we will construct and analyze mouse models that lack proteins required for mitochondrial fusion.